It concerns the production of an "optic contact" at the end of a fibre, that is to say to provide this end with a connection terminal which will then be introduced into different types of connectors. In particular, for the connection of two fibres between themselves, the respective two optic contacts as provided will be joined in a common base having two female positions opposite each other receiving the terminals and permitting the bringing into contact of the end faces of the fibres.
The problem posed by the connection of two optic fibres consists essentially of bringing into coincidence the optic axes of the cores of these, for obtaining the lowest possible transfer loss.
In the case of a connection by joining of two optic contacts, it concerns the preparation individually of each fibre termination with an optimal centering of the core of the fibre on a reference axis of the terminal. If the cores are perfectly centered in the terminals, then alignment faults will result only from mechanical tolerances of the terminal and of the base and from residual play between the elements, faults which can be reduced to very low values (of the order of 0.2 to 0.3 .mu.m) with the better technologies presently available.
The difficulty comes from the fact that it in fact concerns the alignment of two fictional axes, not materialised, and that the alignment of optic sheaths which surround the cores of the two fibres does not necessarily provide the alignment of the cores, insofar as a slight eccentricity always exists in practice between the core and the optic sheath of a given fibre.
This inconvenience is not particularly awkward in the case of multimode fibres, that is to say of fibres of which the diameter of the core is much greater than the wavelength propagated; for example for a 50/125 fibre (diameter of the core 50 .mu.m and the diameter of the optic sheath 125 .mu.m), a misalignment of the cores of 3 .mu.m producing a transfer loss of approximately 0.5 dB.
On the contrary, in the case of monomode fibres (that is to say having a diameter of core (for a fibre with an index step) or an equivalent diameter of core (for a fibre with an index gradient) of the same order of size as the propagated wavelength), for arriving at the same value of transfer loss, it is necessary to attain a precision of alignment of the order of 0.3 .mu.m.
Now no present fabrication of monomode fibres guarantees such concentricity between the axis of the core and the external surface of the optic sheath; values of tolerance generally observed are of the order of+1 to 5 .mu.m.
The solutions proposed up to now for centering with precision a termination of an optic fibre in a connection terminal - particularly in the case of a monomode fibre - provide injection of light into the fibre, either through the other end of the fibre, or through a region close to the end to be connected, which is curved for injection of an optic signal. After transmission through the fibre, the signal is observed through a microscopic viewer, and the termination of the fibre is micromanipulated by the operator until optimal centering is obtained of the luminous spot (corresponding to the core of the fibre) on the reticule of the viewing system.
These known methods of centering have however a certain number of inconveniences, in particular the fact that the quality of the manipulation depends upon the dexterity of the operator.
Further, for opperating the first method (injection from the other end), the connector has to be mounted in a laboratory or in a factory dedicated to the production of fibres, and a splicing on site of the fibre as such will be necessary.
In the case of the second method, the necessity of injecting an optic signal whilst curving the fibre creates the risk of the application of excessive stresses in the fibre; further, it is necesary to lay this bare in the case where it is provided with an opaque mechanical sheathing covering the optic sheath. Further, precise work on the ground is always made difficult because of the poor accessability of elements, ambient pollution, and bad conditions of work, or the like.